In measuring the basis weight of moving sheet material a radiation source is mounted on one side of the sheet material and on the other side is a receiving transducer. The pairs of transducers are then scanned in a cross direction (perpendicular to the movement or machine direction of the sheet material) to provide a cross direction measurement of that particular characteristic.
When a nuclear gauge is used, such as with radioactive krypton gas, such nuclear source must be appropriately shielded when not in use. U.S. Pat. No. 5,432,353 illustrates one technique of shielding where the encapsulated nuclear source is mounted for rotation about a horizontal axis (parallel to the moving web) and when not in use merely rotated to face a side wall of its enclosure. See also U.S. Pat. No. 5,233,195. Since it is desirable to decrease the air gap between the radiation source and the moving web, this source should be as close to the sheet as possible. Thus, normally there is no room for a shutter to shield the source and this was the reason for the mounting in the above patents. Such shielding of the source by rotating it is space-saving. However, where a source other than krypton, such as promethium, is used, which has a much larger area of radiation and thus the encapsulated cylinder is larger, it would be too space consuming to mount such a source as illustrated in the above patents, and still provide proper shielding and also space for a standardization flag.
Also as discussed above, in regard to the alignment between the radiation source and the detector, increasing demands for accuracy of measurement requires some type of error compensation for mechanical imperfections in the transfer mechanisms causing the radiation source and detector to scan across the paper in a cross direction. One form of correction is illustrated in U.S. Pat. No. 5,010,766, in the name of the present inventor and assignee. In that technique a separate unit senses the misalignment and then makes appropriate compensation.